Patent GB-957540-A (GB)
Title: Improvements in or relating to hydraulic control systems
IPC: F15B13/08
Filed: 1962-05-08
Abstract: 957,540. Hydraulic apparatus. INTERNATIONAL HARVESTER CO. May 8,1962 [June 20, 1961], No. 17622/62. Heading F1P. An hydraulic control system of the closed centre type comprises a source P of fluid under pressure, a reservoir 5, at least one control valve 14 for directing flow of fluid to an hydraulic unit 2 for operation of a mechanism, such as a dozer, scraper or the like on a vehicle of the crawler tractor type, and an unloading valve 9 having a valve member 54 defining first and second chambers 52, 47a, the first chamber communicating with the source and the second chamber communicating with the first through a restricted variable orifice 56 and with a pilot line 9a, blocking of the pilot line when the control valve is moved to a position effecting operation of the hydraulic unit building up pressure in the second chamber to close the unloading valve so that pressure from the source is available to operate the unit and mechanism, and unblocking of the pilot line when the control valve is moved to a non-operative position releasing pressure from the second chamber so that the unloading valve opens and by-passes fluid from the source to the reservoir. In the embodiment described, three control valves 14, 15, 16 are provided for controlling hydraulic units 2, 3, 4, respectively, fluid being supplied to the valves by conduits 11, 12, 13 connected to the pump P. Each control valve has lands 19a, 19b, 19c and 19d and is movable to "float", "lower", "raise" and "hold" positions for its associated unit. The valves are connected by conduits 28, 29, 30 and 27 to a reservoir 5, and a passage 9b in the valve 16 is at one end, connected to the return line 27, and, at the other end, connected to a groove 20b in the valve. When the valves are in the "float" or "hold" positions, the groove 20b is connected to a groove 20a which is connected to the pilot line 9a leading to the chamber 47a of the unloading valve by similar grooves in the valves 15, 14. The unloading valve has a hollow member 54 slidable in a sleeve 48 having ports 50 connected to an annular cavity 49 which co-operates with a slot 56 in the member 54 to form the variable orifice. When the member 54 is in its upper position abutting a cover 46, the flow from the pump 8 passes through a conduit 8 to the chamber 52 and through openings 63a to a drain chamber 64 and a passage 31 leading to the reservoir to unload the pump. In its lower position, the member 54 engages a seating 55 to prevent flow to the chamber 64 so that the maximum pressure, as determined by a relief valve 17 is available to operate the units. When the valves 14, 15, 16 are on their "float" or "hold" positions, the chamber 47a is connected by the grooves 20a, 20b in the valves to the reservoir and the pressure from the pump acting across the restriction 56 raises the member 54 to permit the pump to discharge through passage 31 into the reservoir. Upon initial movement of one of the valves, from the hold or float position, the pilot line 9a is restricted an amount determined by notches 33 or 34 and the pressure in chamber 47a rises to move the member 54 downwardly to restrict the flow to the drain chamber and increase the pressure supplied by the pump. Further movement of the valve directs the fluid to one or other end of the unit to be operated, notches 40a or 40b ensuring that only a small flow passes at first to the unit. With further movement of the valve, the pilot line is eventually closed and the member 54 moves to engage the seating 55 and permit the full pressure from the pump to act on the unit.